Various devices heretofore have been used to monitor or detect activity downhole in a well. Many if not all of these devices require the use of a sensor, or logging tool, that is lowered into the well. Depending on the application, this may, for example, add to the cost of drilling the well or may interfere with normal production of the well being monitored. Consequently, it would be desirable to provide for monitoring of activity downhole in a well without requiring intrusion into the well, i.e., the lowering of a measuring device into the well being drilled or the well being monitored. Moreover, in the case of a producing well, it would be advantageous to be able to monitor downhole activity without having to cease well production.
One specific area of interest is the avoidance of well intersections during drilling of new wells in the vicinity of existing and especially producing wells.
As an oil and gas field matures, the density of drilled wells increases as new wells are drilled to fully develop the field. The density of drilled wells may also be high because of the location of the drill site or permitting. Offshore production facilities are by design very high well density areas, as are some environmentally sensitive areas because of the restricted areas allocated for the drill site. Directional wells are often used to maximize the production of the reservoir formation. To optimize production, directional wells are often drilled such that their paths cross or pass in close proximity to one or more existing wells at one or more points along their lengths.
The close intersections that result from drilling in close proximity to existing producing well pose a problem. If the new well is drilled into an existing producing well, high pressure hydrocarbons from the producing well may follow the drill path of the new well or the casing of the producing well to the surface and result in a "blow-out". Undesirable pressurization of upper formations has also occurred.
Various procedures have been devised to eliminate the possibility of blow-outs or other undesirable consequences of an intersection. A common procedure is to temporarily plug the existing producing well or wells within close proximity of the new well being drilled at least until the new well has been drilled past the point of possible intersection. The temporary plugging operation, however, is expensive and production is deferred during the "shut-in" period. At times, the temporary plugging operation, or "safe-out", may be avoided by designing and drilling the well in a more expensive manner by steering a course far enough away from the existing wells. This latter approach, however, is undesirable because of high cost and in many instances it is difficult if not impossible to implement due to close spacing of wells at the surface, such as exist on offshore platforms and drill pads.
The above problem was addressed by Hoehn, Jr. in U.S. Pat. No. 4,593,770. This patent describes a method for preventing the drilling of a new well into a production well be using a logging sonde to inject alternating current into the well casing of the producing well or wells at desired subsurface depth points. The resulting magnetic fields created around the well casings of the producing wells are detected by a magnetometer in a new well being drilled to prevent drilling into one of the production wells.